1. Field of the Invention
The present invention relates generally to a radio communication system, and, more particularly, to a radio communication system that uses an OFDMA (Orthogonal Frequency Division multiplexing Access) connection mode and a base transceiver station apparatus using the same.
2. Description of the Related Art
Recently, an OFDMA (Orthogonal Frequency Division multiplexing Access) system is applied to a mobile communication system that is one form of radio communication systems.
In the OFDMA, as shown in FIG. 1, for example, a 20-MHz band is divided into a plurality of consecutive 5-MHz sub-carriers, for example, in the frequency direction, and each sub-carrier is assigned to a user or service to achieve user- or service-multiplexing.
In FIG. 1, consecutive sub-carriers are assigned to four users 1 to 4. Since a plurality of consecutive frequencies is collectively assigned to a plurality of users, this multiplexing method may be referred to as localized OFDMA.
As shown in FIG. 2, the distributed OFDMA is also proposed to select one or a consecutive plurality of sub-carriers at certain frequency intervals to create sub-carrier groups, which are assigned to users.
In such a case, a radio channel used by each user is constituted by a data channel (DPDCH) and a control channel (DPCCH), which are mapped to one radio frame format. The frame format of the radio channel is fixed.
A configuration shown in FIG. 3 is a typical base transceiver station configuration in the mobile radio system, which is the radio communication system using the OFDMA. A control signal on the control channel (DPCCH) from a control signal generating unit 30 and a data signal on the data channel (DPDCH) from an upper layer are mapped to a predetermined format by a frame format generating unit 31.
The output from the frame format generating unit 31 is encoded by an encoding unit 32, is modulated by a modulating unit 33 in a modulation mode such as QPSK, is subjected to frequency conversion to a radio frequency signal by a transmission radio unit 34, and is emitted from an antenna 35.
FIG. 4 is a configuration example of a transmission radio unit 34 and, particularly, is a configuration example of an OFDM radio unit. The output of the modulating unit 33 is converted by an S/P converter 340 to a parallel signal. The parallel signal is subjected to an IFFT process by an inverse Fourier transform (IFFT) circuit 341 and is restored to a serial signal by a P/S converter 342. This serial signal is added with guard intervals GI, is converted to a radio frequency by a frequency converter 345, and is emitted from the antenna 35.
FIG. 5 is a configuration example of a terminal corresponding to the configuration of the base transceiver station of FIG. 3. A received signal received by a reception antenna 40 is subjected to frequency conversion to be a base band signal and is input to a demodulating unit 42. The signal is subjected to a demodulating process corresponding to the modulating unit 33 of the base transceiver station by the demodulating unit 42 and is divided into the data and the control signal by a signal dividing unit 44.
On the other hand, in FIG. 5, received power is measured from a pilot signal and the result thereof is sent to a channel estimating unit 46, etc. The channel estimating unit 46 estimates a propagation path, etc. from the measured power.
In this way, when the terminal performs the measurement of the received power, the channel estimation, the power control, the adaptive modulation control, etc. by receiving the pilot channel or pilot symbol included in the control channel (DPCCH), the intended control is nor performed accurately, unless the transmission quality is ensured at a certain level or more (e.g., an error rate of 1.0E to 2).
To ensure the transmission quality, the transmission power of the control signal on the control channel (DPCCH) is made higher than the case of the usual data transmission.
FIGS. 6A and 6B are diagrams for describing problems to be solved by the present invention. In FIGS. 6A and 6B, FIG. 6A shows a time-frequency distribution of four-user sub-carriers mapped to a radio frame format. The frame format of the radio channel is fixed. Since the control channel (DPCCH) is control information necessary for communication, the transmission quality thereof must be made higher than the data channel (DPDCH).
Therefore, as shown in a transmission power distribution of FIG. 6B, countermeasures are taken in a W-CDMA (Wideband Code Division Multiple Access) system by making the transmission power of the control channel (DPCCH) higher than the data channel (DPDCH). From FIG. 6B, it is understood that the transmission power is made higher at the cycle of the control channel (DPCCH).
In this way, the transmission power of the control signal on the control channel is increased conventionally. As shown in FIGS. 3 and 4, since the OFMDA base transceiver station performs the IFFT, it is desirable that the beginnings of the data for the users, i.e., the beginnings of the radio frames are identical.
If the beginnings of the radio frames are not identical, since the signal process becomes complicated, the apparatus configuration and the control are also complicated. Therefore, the beginnings of the radio frames must be identical.
However, if transmission for a plurality of users is performed concurrently with the use of the fixed radio frame format as described above, the positions of the control channels (DPCCH) are identical (see FIG. 6A). Therefore, since the transmission signal for each user is added in the same phase in the OFDMA, a peak of the transmission power is generated at the time of the transmission on the control channel (DPCCH) with high transmission power. For example, in FIG. 6B, P1 is peak power at a time T1.
Consequently, a peak-to-average power ratio (PAPR) becomes a higher value, which generates various disadvantages.
In general, the following problems are generated when the PAPR is a higher value.                In the design of an amplifier of the radio apparatus, a margin of the design must be made greater.        Since the peak output power becomes higher power, the efficiency of the amplifier is deteriorated.        Since the peak output power becomes higher power, power consumption is increased.        
Because of these problems, it is important to constrain the PAPR to a lower level. In the W-CDMA system, since the positions of the control channels are identical and a greater peak is generated in the transmission power as described above, it is important to constrain the PAPR to a lower level.
For such problems, it is proposed to perform encoding with the use of codes with different code distances, which is one of an unequal error protecting method, to control such that transmission peak power of a multi-carrier modulation signal becomes a predetermined value or less (Japanese Patent Application Laid-Open Publication No. 2000-286818).
An invention described in Japanese Patent Application Laid-Open Publications No. H11-154904 and 2005-57610 are an invention relating to the transmission power/PAPR constraint.
An invention described in Japanese Patent Application Laid-Open Publication No. H11-154904 is intended to reduce the peak-to-average power ratio by time-multiplexing the pilot signal and the data signal to form channels and to reduce the number of orthogonal symbols used for forming the channels. The invention described in Japanese Patent Application Laid-Open Publication No. 2005-57610 is intended to sort the order of generated diffusion codes randomly to determine the order corresponding to each of a plurality of users.
3GPP Contribution (R1-050604), Sophia Antipolis, France, 20-21 Jun. 2005 proposes a format that is dispersedly arranged to be a common control channel and this is applied to the common control channel and high-speed users in scheduling dependent on a frequency channel.
However, in the invention described in Japanese Patent Application Laid-Open Publication No. 2000-286818, since codes must be selected to vary the minimum code distances, the code selection is restrained. Japanese Patent Application Laid-Open Publication Nos. H11-154904 and 2005-57610 do not refer to the increasing of the PAPR due to the accumulation of the power of the control channel. Therefore, the dispersed arrangement is not indicated which is performed by paying attention to the control channel. Although 3GPP Contribution (R1-050604), Sophia Antipolis, France, 20-21 Jun. 2005 describes that the dispersed arrangement is performed such that the transmission characteristics are improved at the time of high-speed movement, it is not indicated that the dispersed arrangement is performed to reduce the PAPR.